Defocus distance and aperture

Hi all,
I do not know too much about photography like you but I do like using small aperture value (1.7, 2.8) to make the background always looking defocused.
There is something though which I do not understand is what we exactly mean by defocus distance and how larger and smaller aperture affect that?

Tomorrow I will try once more the kit lens that gives only 3.5 aperture what my defocus distance would be and how should I position my subject so the background looks defocsed?

Hi Alex. I've never heard of the term "defocus distance" before. But if there is such a thing, someone else here may chime in and clue us in.

But if I'm understanding your question, you're trying to figure out much space to put between your subject and the background to get an out-of-focus background, correct?

Well, there's no simple answer to that. The focal point (usually where you place your subject) is a point in space that you can calculate with any given lens/aperture combination, and every other point in space will be less in focus. The farther you get from the focal point, the more objects in the frame will be "out of focus."

Here's where it gets a little confusing. Though you can focus on a point in space, more than just that point in space will be "in focus." Depending on the lens/aperture combo, you can have a very narrow "in focus" area, or a wide one. But everything outside of that area (which is usually called depth-of-field) will be out of focus, and the farther you place the background from the in-focus area, the more the background (or foreground) will be out of focus.

So there's no way to calculate a defocus distance, because the level of defocus changes as you move away from the "in-focus" area. There's no exact point in space where things suddenly change from in focus to out of focus. It's a gradual shift.

As an example, with your kit lens set at 18mm with an aperture of f/3.5, if your subject (or focal point) is at 2 meters, then everything from the camera to 139cm, and everything from 354cm to infinity will be out of focus. What's in between those distances will be in focus. Of course, objects 100 meters behind the subject will be far more out of focus than objects 10 meters behind the subject.

Does that explain why there's no "defocus distance?"

There are many "depth of field" calculators available in the Internet. Just Google that term, and you'll find one. They may help you get what you're after.

What you'll also note is how depth of field is reliant on focal length too, not just aperture. Traditional focal lengths for portraits lie somewhere in the 50-100mm focal length as wide angles cause unusual distortions in peoples faces [for example, see here: How Lens Focal Length Shapes the Face & Controls Perspective: A Lighting Tutorial « Education Blog]. If you shoot a subject 10 feet away at 18mm f/5.6 you'll get an infinite depth of field, whereas if you zoom to 55mm staying at f/5.6 the depth of field becomes only 2.3 feet.

So if your objective is to defocus the background, don't forget to consider your focal length and step back a bit as well as aperture.

One thing that works, even on lenses which don't have large minimum apertures, is to place the camera as close as possible to the object and the background as far away as possible. Then you will get a nice background blur (unless you use a wide lens with a very small aperture).

But actually the best way is to learn how each lens works by taking practice shots. It doesn't take long to learn the characteristics of a lens.